Biaxially oriented polypropylene (BOPP) films used for packaging, decorative, and label applications often perform multiple functions. They must perform in a lamination to provide printability, transparent or matte appearance, or slip properties; sometimes they also must provide a surface suitable for receiving organic or inorganic coatings for gas and moisture barrier properties; and sometimes must provide a heat sealable layer for bag forming and sealing or a layer that is suitable for receiving an adhesive either by coating or laminating.
However, in recent years, interest in “greener” packaging has been developing strongly. Packaging materials based on biologically derived polymers is increasing due to concerns with non-renewable resources, waste production, raw materials, and the production of greenhouse gases. Biodegradable polymers would alleviate the growing environmental problem of the production of an excessive amount of plastic waste. Non-biodegradable plastic waste requires years to decompose and comprises an ever-increasing volume fraction of the waste present in landfills. Also, it is believed that bio-based polymers, once fully scaled up, will help to reduce reliance on petroleum and thereby reduce the production of greenhouse gases due in part to their sustainably-sourced feedstocks (i.e. plant-sourced).
Bio-based polymers such as polylactic acid (PLA), which is derived from corn starch and thus can be considered to be derived from a renewable resource, is one of the more popular and commercially available materials available for packaging film applications. However, due to the commercial expense of bio-based polymers compared to traditional polymers and the difficulties that can arise in the processing of these biopolymers to form a product comparable to or matching that of existing products, there has been little commercial success. Many compositions involving these polymers exhibit limited quality, processability, degradability, or some combination thereof.
U.S. Pat. No. 5,153,074 describes the use of an extrusion-grade EVOH of typically 48 wt % ethylene content coextruded with a maleic anhydride-grafted propylene homopolymer or copolymer and biaxially oriented into a film. This film is then metallized on the EVOH surface for high barrier properties. However, such a film formulation is not degradable or sustainably-sourced. Nor is a biopolymer substrate such as PLA contemplated. In addition, the high ethylene content of the EVOH used may prevent such a material from being easily biodegradable or compostable.
U.S. Pat. No. 5,175,054 describes the use of in-line coating between the Machine Direction Orientation (MDO) and Transverse Direction Orientation (TDO), a PVOH dispersion blended with a metal-containing ionic copolymer onto a biaxially oriented polymer substrate. The ionic copolymer acts as a tie-layer resin or primer to enable good adherence of the PVOH to the polyolefin substrate. This in-line coated film is then metallized via vacuum deposition on the PVOH blend surface. However, such a structure provides little improvement in oxygen barrier properties compared to current metallized BOPP films. In addition, this reference is not directed to a biopolymer-based substrate and the films would not exhibit biodegradable or compostable properties.
U.S. Pat. No. 4,464,438 describes the blend of PVOH and EVOH with a processing aid to enable extruding and stretching such a blend into a film. However, these blends were not co-extruded or coated onto a BOPP substrate, nor is a biopolymer-based substrate contemplated.
U.S. Pat. No. 5,731,093 describes the use of an in-line coating between the MDO and TDO, a PVOH blend with polyvinylidene chloride (PVdC) onto a multilayer biaxially oriented polypropylene film substrate. The PVOH/PVdC blend's surface is then metallized via a vacuum deposition process. Excellent barrier properties are obtained, but the use of PVdC raises environmental concerns. Moreover, the polypropylene substrate would not be biodegradable.
U.S. Pat. No. 5,473,439 describes the use of crosslinked EVOH coatings on biaxially oriented polypropylene or polyethylene substrates. However, there is no indication of the efficacy of such coatings on PLA substrates. In addition, these polyolefin substrates would not be biodegradable.
U.S. patent application Ser. No. 12/332,153 describes coextrusions of polyolefin metal receiving layers on a PLA core layer to improve moisture barrier properties after metallization. However, no vinyl alcohol polymer layers are contemplated to be applied directly to the PLA substrate and the polyolefin-based metal receiving layers would not be compostable or degradable.
PCT Application PCT/US2009/54022 describes the improvement of moisture barrier properties on metallized PLA substrates via a unique process of sputter-deposited copper or other metal “seeding” or “priming” of the PLA substrate prior to aluminum vapor deposition. However, this process is not contemplated on a vinyl alcohol polymer layer applied to the PLA substrate. This reference is incorporated in its entirety in this application.
U.S. patent application Ser. No. 12/890,349 describes the improvement of moisture barrier properties of metallized PLA films by using a variety of coatings applied to the PLA substrate either via in-line or off-line coating processes including the use of EVOH/PVOH aqueous coating blends with a crosslinker. This reference is incorporated in its entirety in this application.
PlasticsToday.com article “Film Extrusion: Bioplastic Barrier Film Matches EVOH, PA” by Matt Defosse, published 17 Jul. 2009, http://www.plasticstoday.com/print/31263, describes blown and cast film products made by Colines SpA using PLA/PVOH coextrusions. However, these films are not biaxially oriented, are very thick (50-90 μm), are not metallized, and are designed for oxygen barrier and not moisture barrier.
TAPPI 2010 Conference (Apr. 18-21, 2010) presentation “Coextruded Film Structures of PLA and EVOH,” by Kuraray America Inc., described blown and thermo-formed films of EVOH and PLA coextrusions for food packaging containers focusing on oxygen gas barrier properties. The presentation also only considers film structures in which the EVOH layer is a core layer encapsulated between two skin layers of PLA and not as a surface skin layer on a PLA core layer. Metallization of the structure was also not contemplated, nor as a biaxially oriented film.